Piezoelectric flexural transducer and use thereof

ABSTRACT

A piezoelectric flexural transducer having an elongated support body ( 2 ) which on at least one longitudinal side is provided with a piezoelectric unit ( 5 ). The piezoelectric unit ( 5 ) has one or more piezoelectric material layers ( 12 ) and electrodes ( 13   a  and  13   b ) associated with same. Furthermore, the piezoelectric unit ( 5 ) includes an electrically insulating coating surrounding the body ( 8 ) right round it, the section ( 26 ) of the coatings lying between the piezoelectric body ( 8 ) and the support body ( 2 ) being provided with one or more electrically conductive vias ( 27   a  and  27   b ). The latter provide the necessary electrical connection between the electrodes ( 13   a  and  13   b ) and the support body ( 2 ).

FIELD OF THE INVENTION

The invention relates to a piezoelectric flexural transducer comprisingan elongated support body which on at least one longitudinal side isfitted with a piezoelectric unit, which has a piezoelectric body withone or more piezoelectric material layers and electrodes associated withsame, the electrodes being connected in an electrically conductingmanner with the support body. Furthermore, the invention relates to useof such a piezoelectric flexural transducer.

BACKGROUND OF THE INVENTION

A prior art piezoelectric flexural transducer of this type in accordancewith the patent publication WO 99/17383 possesses an elongated supportbody, which at opposite longitudinal sides is fitted with a respectivepiezoelectric unit. The piezoelectric unit comprises a piezoelectricbody manufactured using multi-layer technology and possessing aplurality of piezoelectric material layers and electrodes associatedwith same. By the application of a drive voltage to the electrodeselectric fields may be produced in the piezoelectric body, this causinga longitudinal contraction of the piezoelectric body and furthermore adeflection, comparable with a pivoting motion, of the piezoelectricflexural transducer in relation to the position at which thepiezoelectric flexural transducer is held. In order to be able to applythe drive voltage to the electrodes of a respective piezoelectric unitare in electrical contact with the support body, which is able to beconnected with a source of voltage.

During operation of such known flexural transducers moisture mayengender problems. If the piezoelectric flexural transducer is forexample employed as the setting member of a valve for controlling fluidflow, and is at all times surrounded by pressure medium containingmoisture, it is possible for partial short circuits to occur, whichimpair the deflection characteristics of the piezoelectric flexuraltransducer and the functional reliability of the means fitted with thepiezoelectric flexural transducer.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a piezoelectricflexural transducer of the type initially mentioned which while allowingsimple manufacture is not sensitive to external effects and moreparticularly is insensitive to moisture. A further aim resides in theuse of such a piezoelectric flexural transducer.

The first mentioned object is to be attained by a piezoelectric flexuraltransducer of the type initially mentioned in the case of which thepiezoelectric unit comprises an electrically insulating coatingsurrounding and encasing the piezoelectric body, such coating beingprovided with one or more electrically vias for making electricalcontact with the electrodes.

It is in this manner that the invention provides a piezoelectricflexural transducer, in the case of which the piezoelectric body havingthe electrodes is completely encapsulated by the electrically insulatingcoating surrounding it and is screened off from the outside. Thepiezoelectric body and its component parts are therefore protectedagainst external influences and functional impairment such as that dueto moisture, is not possible. For the application of the drive voltagethe piezoelectric unit is consequently provided, in accordance with itsparticular design, with one or more electrically conductive viasextending through the coating.

It would in principle be possible to provide the entire piezoelectricflexural transducer with a uniform coating. However, the separatecoating of one or more already existing piezoelectric unitsindependently of the support body has substantial advantages both fromthe point of view of manufacturing technology and also as regards costs.The vias can produce the electrically conductive connection between theelectrodes of the piezoelectric material and the conductors of thesupport body or lead to electrodes on other outer sides of thepiezoelectric material.

Further advantageous developments of the invention are defined in thedependent claims.

It is convenient for the vias to be located in the section of thecoating, facing the support body, so that on the application of apiezoelectric unit to the support body electrical contact between arespective piezoelectric unit and the support body is madesimultaneously. Furthermore, the adhesive layer normally present betweenthe piezoelectric unit and the support body and serving for securing thepiezoelectric unit may reliably seal off the surrounding part of thevias from the outside.

At each respective via the support body preferably possesses anelectrically conductive contact face connected with the via. It is alsoan advantage for each electrically conductive via to be provided with acontact layer on the outer face (which faces the support body) of thesection, which rests on the support body, of the coating, such layerserving to make contact with the support body. It may be made with asufficiently large periphery so that no complex adjustment work isnecessary for mounting the piezoelectric unit with a reliable and securecontact making effect.

The structure in accordance with the invention is also advantageous evenwhen the piezoelectric body has only one piezoelectric material layer,that is to say is made in the form of a monolithic piezoelectric body.Owing to the plurality of electrodes present such structure is howeverparticularly to be recommended in the case of a multi-layerpiezoelectric body with a plurality of piezoelectric material layers.

As a material for the electrically insulating coating it is preferred toemploy a parylene (generic term for thermoplastic polymer films, whichare formed from p-xylene) or a polyimide (generic term for hightemperature stable plastics, which generally contain aromatic groups).

The vias may be formed in any desired manner. In the case of oneparticularly simple design through holes are machined in the coatingduring the manufacture thereof and such holes are then filled with aconductive material, as for instance a conductive adhesive.

In principle the support body may be provided with its own coating.Since in the form of the piezoelectric body however the relevantcomponents are also encapsulated, a coating-free design of the supportbody is readily possible as well.

The piezoelectric flexural transducer may as a unimorphous flexuraltransducer be provided with only one piezoelectric body or as atrimorphous flexural transducer with two encapsulated piezoelectricbodies arranged on opposite longitudinal sides.

To achieve the above mentioned further aim the use of the piezoelectricflexural transducer as a setting member having a drive or sealingfunction is provided for, for example in conjunction with a valveserving for fluid control. The outer face of the coating may heredirectly serve as the actuating face and/or sealing face and in thisconnection may consist of a material with the desired coefficient offriction and/or sealing properties.

In the following the invention will be explained in detail withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal section taken through a preferred embodiment ofthe piezoelectric flexural transducer in accordance with the inventionin a highly enlarged view on the section line I—I of FIG. 2.

FIG. 2 is a plan view of the piezoelectric flexural transducer of FIG.1, partly in a longitudinal section taken on the line II—II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The piezoelectric flexural transducer generally referenced 1 in FIG. 1comprises a plate-like, elongated support body 2 having a sheet-likeconfiguration and made in one or more layers. A single layer design isillustrated, which preferably consists of a composite fiber material,for example an epoxy resin which is reinforced with carbon fibers orglass fibers. A further preferred design has a multi-layer structure ofplastic, for example polyimide. An electrode structure may be containedin this material.

Such a possible multi-layer design is indicated in FIG. 1 by a chainedlayer transition or demarcation line 3, same simultaneously indicatingthe plane of extent of the support body 2.

One of the two outer faces with a large area on the longitudinal sidesof the support body 2 is depicted as the first support face 4, which inthe front part is provided with a first piezoelectric unit 5. The latteralso has an elongated configuration and runs in parallelism to thelongitudinal axis of the support body 2, it however preferably beingsomewhat shorter than the support body 2 so that a connection section 6of the latter protrudes on the back side past the first piezoelectricunit 5.

The support body 2 is provided with a plurality of electrical conductorsextending in its longitudinal direction, such conductors herein beingtermed first and second electrical conductors 7 a and 7 b for bettercomprehension. They run from the terminal portion of the connectionsection 6 toward the first piezoelectric unit 5 underneath same. At theconnection section 6 the first and the second electrical conductors 7 aand 7 b can be connected in a known manner with a power supply circuitto produce or conduct the drive voltage necessary for the operation ofthe piezoelectric flexural transducer 1. The first and the secondelectrical conductors 7 a and 7 b in the working embodiment run on thefirst support face 4.

The first piezoelectric unit 5 mounted on the first support face 4possesses an elongated piezoelectric body 8, which has severalpiezoelectric material layers 12 extending flatwise and superposed in astacking direction 9 at a right angle to the plane 3 of extent. Thepiezoelectric material is for example a suitable piezoelectric ceramic.Between piezoelectric material layers 12 which are directly adjacent inthe stacking direction 9 alternating sheet-like first and secondelectrodes 13 a and 13 b extend. They may be formed as metallizedlayers, for example in the form of a silver and/or palladium metallizedlayer. Apart from the bottom piezoelectric material layer 12 a adjacentto the support body 2 and the top piezoelectric material layer 12 bopposite to same, which are formed without electrodes on the sidesfacing away from the multi-layer body, accordingly the piezoelectricmaterial layers 12 are each flanked on opposite sides by a first and asecond electrode 13 a and 13 b.

The first and the second electrodes 13 a and 13 b project terminally outfrom the piezoelectric material layers 12, all first electrodes 13 abeing jointly linked with first electrical contact means 14 a, whereasthe second electrodes 13 b are in contact with second electrical contactmeans 14 b.

With the first piezoelectric unit 5 mounted the first electrical contactmeans 14 a and accordingly the first electrodes 13 a are connected withthe first conductor 7 a of the support body 2. In a similar mannerelectrical contact is produced between electrodes 13 b by way of thesecond electrical contact means 14 b with the second electricalconductor 7 b.

The first piezoelectric unit 5 is secured in some suitable way to thefirst support face 4. In the working embodiment illustrated this isensured by bonding, an adhesive layer 15 being applied between thebottom side of the first piezoelectric unit 5 and the first support face4.

If a drive voltage at a suitable level is applied by way of first andthe second electrical conductors 7 a and 7 b to the first and the secondelectrodes 13 a and 13 b, electric fields polarized in the stackingdirection will be formed within the piezoelectric material layers 12,such fields having the effect that the length dimensions of thepiezoelectric material layers 12 as measured in the length direction 16of the piezoelectric flexural transducer 1 will be become shorter. Theresult of this is that the piezoelectric flexural transducer 1 will bebowed, this meaning a deflection perpendicularly to the longitudinaldirection of the flexural transducer as indicated by the arrow 17. Thismovement may be employed to perform a drive or sealing function.

In practical use the piezoelectric flexural transducer 1 is held on therear side, it performing the deflection movement at its front part. Theflexural transducer may be held at the connection section 6 and/or inthe adjoining rear terminal part of the first piezoelectric unit 5.

A preferred application of the piezoelectric flexural transducer 1 isillustrated in chained lines in FIG. 1. It will be seen that there ishere a diagrammatically indicated section of the housing 18 of a valve,which has a fluid duct 22 with a fluid duct opening 23 turned toward thetop outer face 24 a (facing away from the support body 2) of the firstpiezoelectric unit 5. The piezoelectric flexural transducer 1 may now beemployed for the control of a fluid flow by opening or closing the fluidduct opening 23 dependent on the control state. The piezoelectricflexural transducer 1 consequently here implements a sealing function.

Other applications of the piezoelectric flexural transducer arepossible. It may for example perform a drive function, it then acting asa setting member on a component of any type which is to be moved, itagain possibly being a question of a valve component.

The piezoelectric body 8 could also have a single layer structureinstead of the depicted multi-layer structure, the body 8 then onlycomprising a single piezoelectric material layer with a correspondinglygreater thickness so that the piezoelectric material would have amonolithic structure.

The first piezoelectric unit 5 is furthermore characterized by having anelectrically insulating coating 25 surrounding the piezoelectric body onall sides, that is to say completely. All piezoelectric material layers12 and electrodes 13 a and 13 b are accordingly encapsulated by acoating as a sort of casing and shut off from the outside surroundings.As a result there is optimum protection against any moisture or otherdamaging effects.

The piezoelectric body 8 of the piezoelectric unit 5 is furthermore notin direct contact with the support body 2, which is itself best designedfree of any coating. For attachment on the support body 2 the abovementioned adhesive layer 15 is applied between the first support face 4and the bottom outer face 24 b, facing it, of the bottom coating section26 extending between the support body 2 and the piezoelectric body 8.

For making electrical contact between the first and the secondelectrodes 13 a and 13 b and the first and second electrical conductors7 a and 7 b, provided on the support body 2, the first piezoelectricunit 5 is provided, preferably in the bottom coating section 26, withelectrically conductive vias 27 a and 27 b which extend through thecoating 25. Within the space enclosed by the coating 25 they areelectrically connected with the first and second contact means 14 a and14 b with the correct coordination and outside the first piezoelectricunit 5 they are in electrical contact with the first and secondelectrical conductors 7 a and 7 b.

The first and the second vias 27 a and 27 b are so structured that theyprovide for a fluid-tight sealing off of the interior space delimited bythe coating 25 and containing the piezoelectric body 8. In the workingembodiment this is achieved since the coating 25 has through openings 28at the vias 27 a and 27 b, such through openings being filled e.g. witha conductive adhesive or an electrically conductive plastic material,the material providing for an autogenous connection with the material ofthe coating 25.

Since the coating 25 consists of insulating material, and preferably adielectric material is employed, short circuits or other impairments maybe avoided and more particularly additional electrical insulating meansare unnecessary as regards of the electrodes 13 a and 13 b covered overby the coating 25, and of the electrical contact means 14 a and 14 b.

A coating 25 of a material which has proved particularly suitable is oneof parylene polymer film (parylene generically denoting thermoplasticpolymer films produced from p-xylene). More particularly, use is made ofso-called parylene N (unsubstituted poly-para-xylylene), parylene D(substitution of two aromatic H atoms by two chlorine atoms) or paryleneC (substitution of one aromatic H atom by one chlorine atom), the lastnamed material being preferred owing to its electrical and physicalproperties. More particularly it does possess an extremely lowpermeability as regards moisture and corrosive gases. Parylene polymersmay be produced in the form of structurally continuous, extremely thinlayers so that the overall dimensions of the piezoelectric flexuraltransducer are hardly changed by having the applied coating.

As an alternative to a parylene polymer film consisting of paryleneplastic, as a material for the coating 25 it is also possible to employa polyimide plastic, preferably Kapton as marketed by DuPont.

The manufacture of the plastic coating 25 may for instance be performedas part of a layer deposition process.

In order to ensure that the operation of providing the support body 2with a piezoelectric unit 5 has as little effect on dimensionaltolerances as possible, it is an advantage for each via 27 a and 27 b tobe provided with a contact making layer 32 provided on the bottom outerface 24 b, which faces the support body 2, of the coating section 26,such layer 32 being connected with the respective via 27 a and 27 b,while however having a larger base area than it. The contact makinglayers 32 are indicated in FIG. 2 by chained lines.

In a similar manner it is an advantage for the first and secondelectrical conductors 7 a and 7 b to possess an electrically conductivecontact face 33 at the respective vias to make contact with same, suchcontact face 33 being indicated in FIG. 2 in chained lines. Their basearea may with advantage be larger than that of the contact making layer32 so that the contact making layer 32 may rest with its full area onthe associated contact making face 33. The contact making layers 32 may,as well as the contact making faces 33 and all the electrical conductors7 a and 7 b, be constituted by a suitable metallized layer.

Since the first piezoelectric unit 5 in the working example includes twoelectrode groups 13 a and 13 b and for each electrode group a via 27 aand 27 b is provided, the first piezoelectric unit 5 accordingly has asmall number of only two vias 27 a and 27 b, which are preferably spacedapart in the longitudinal direction 16 of the support body 2. Dependenton the design of the piezoelectric unit furthermore only one single via27, or more than two, could be provided.

The piezoelectric flexural transducer 1 in accordance with example has aunimorphous structure with only one piezoelectric unit 5. Neverthelessthe invention is naturally applicable to flexural transducers 1possessing more piezoelectric units. The design of a trimorphouspiezoelectric flexural transducer illustrated in FIG. 1 may serve as anexample in this respect, the support body 2 having a second support face4 a on the longitudinal side opposite to the first support face 4, thesupport face 4 a being fitted with a second piezoelectric unit 5 a(indicated in chained lines).

A further advantage of the piezoelectric flexural transducer 1 is thatgiven a suitable selection of the material for the coating 25 it ispossible to dispense with additional components in order to design thecontact, necessary for drive and/or sealing purposes, with acounterpart. In this case the outer face of the coating 25 may bedirectly employed as an actuating and/or sealing face, at which thepiezoelectric flexural transducer 1 engages a component to be drivenand/or engages a valve seat.

What is claimed is:
 1. A piezoelectric flexural transducer comprising anelongated support body which on at least one longitudinal side is fittedwith a piezoelectric unit, which has a piezoelectric body with one ormore piezoelectric material layers and electrodes associated with same,characterized in that the piezoelectric unit comprises an electricallyinsulating coating surrounding and encasing the piezoelectric body, suchcoating being provided with one or more vias for making electric contactwith the electrodes.
 2. The piezoelectric flexural transducer as setforth in claim 1, characterized in that the vias are arranged in thesection of the insulating coating lying between the piezoelectric bodyand the support body and produce an electrically conductive connectionbetween the electrodes and the support body and, respectively, theelectrical conductors of the support body.
 3. The piezoelectric flexuraltransducer as set forth in claim 2, characterized in that at each viasupport body has an electrically conductive contact face in electricalcontact with same.
 4. The piezoelectric flexural transducer as set forthin claim 2, characterized in that for each via there is a contact makinglayer provided on the outer face, facing the support body, of thesection, resting against the support body, of the coating, such contactmaking layer preferably being in the form of a connection track, formaking contact with the support body to the electrodes.
 5. Thepiezoelectric flexural transducer as set forth in claim 1, characterizedin that the support body has a single or multiple layer design ormultiple layer design, the drive voltage being able to be applied by wayof electrical conductors of the support body.
 6. The piezoelectricflexural transducer as set forth in claim 5, characterized in that theelectrical conductors are at least partially arranged in the interior ofthe multi-layer support body.
 7. The piezoelectric flexural transduceras set forth in claim 1, characterized by at least two vias spaced apartin the longitudinal direction of the support body.
 8. The piezoelectricflexural transducer as set forth in claim 1, characterized in that thesupport body comprises a fiber reinforced composite body, which moreparticularly consists of carbon fiber reinforced or glass fiberreinforced epoxy synthetic resin.
 9. The piezoelectric flexuraltransducer as set forth in claim 1, characterized in that of the supportbody comprises a polyimide and more particularly has an integratedelectrode structure.
 10. The piezoelectric flexural transducer as setforth in claim 1, characterized in that the piezoelectric body isdesigned in the form of a multi-layer piezoelectric body.
 11. Thepiezoelectric flexural transducer as set forth in claim 1, characterizedin that the coating of the piezoelectric unit, has dielectric materialproperties.
 12. The piezoelectric flexural transducer as set forth inclaim 1, characterized in that the coating comprises a parylene plastic,preferably in the form of a polymer film, and preferably so-calledparylene C.
 13. The piezoelectric flexural transducer as set forth inclaim 1, characterized in that the coating comprises a polymide plasticlayer, preferably consisting of so-called Kapton.
 14. The piezoelectricflexural transducer as set forth in claim 1, characterized in that thecoating is constituted by a plastic layer produced by a layer depositingmethod.
 15. The piezoelectric flexural transducer as set forth in claim1, characterized in that such via comprises electrically conductivematerial introduced into a through opening in the coating, such materialbeing for example a conductive adhesive.
 16. The piezoelectric flexuraltransducer as set forth in claim 1, characterized in that the supportbody is free of coating.
 17. The piezoelectric flexural transducer asset forth in claim 1, characterized in that the piezoelectric unit isbonded to the support body.
 18. The piezoelectric flexural transducer asset forth in claim 1, characterized in that the support body is fittedwith a respective piezoelectric unit on two longitudinal opposite sides.19. The use of a piezoelectric flexural transducer as set forth in claim1 as a setting member with at least one of a drive function and asealing function, the outer faces of its coating being preferablyemployed directly as at least one of an actuating face and a sealingface.